The present invention relates to a hydraulic power steering device, for example, for use in motor vehicles which is responsive to the speed of the vehicle.
Throughout the specification, the right-hand side of FIG. 10 will be referred to as "front," and the left-hand side thereof as "rear."
In the following description, like parts are designated by like reference numerals throughout the drawings.
The specification of U.S. Pat. No. 4,796,715 discloses a vehicle speed responsive hydraulic power steering device which has heretofore been used in motor vehicles. The device is shown in FIGS. 10 to 13. With reference to FIGS. 10 to 13, the conventional power steering device comprises an output shaft 31 disposed within a housing 30 and having a tubular rear portion 31a extending axially of the shaft. The tubular portion 31a is formed with a plurality of holes 32 extending radially through its peripheral wall and arranged circumferentially thereof at a specified spacing. Provided inside the housing 30 is a hollow input shaft 33 inserted at its front portion in the tubular portion 31a of the output shaft 31 and coaxial with the output shaft 31. A torsion bar 34 is provided between the output shaft 31 and the input shaft 33 so as not to rotate relative to these shafts 31, 33 about the axis thereof.
The front portion of the output shaft 31 is integral with a pinion 36 having teeth meshable with the rack of a rack bar 35 for steering the front wheels. The input shaft 33 is connected to a steering shaft carrying an unillustrated steering wheel. Provided around the input shaft 33 is a rotary valve 37 (control valve) having a valve body 37a secured to the output shaft 31. Owning to a relative angular displacement occurring between the input shaft 33 and the output shaft 31 due to the torsion of the torsion bar 34, the rotary valve 37 is adapted to introduce pressure oil, which is forwarded thereto by a hydraulic pump 38 driven, for example, by the rotation of the engine, into the housing 30 via a conduit 39 and an inlet port 40 in the housing 30, to supply the oil selectively to one of the two ports of an unillustrated power cylinder and to control the amount of supply.
A plunger 41 made of a metal material, such as JIS SUJ2 as hardened, is fitted in each hole 32 in the tubular portion 31a of the output shaft 31 and is slidable axially of the hole 32. The end of the plunger 41 closer to the input shaft 33 is integrally formed with a spherical protuberance 41a. Positioned in coincidence with the holes 32 is an annular groove 42 formed in the outer periphery of the output shaft 31 over the entire circumference thereof. A split ring 43 is fitted in the annular groove 42 for restraining the plungers 41 from moving radially outward.
An annular groove 44 is also formed in the inner periphery of the housing 30 over the entire circumference thereof and is opposed to the holes 32. On the front and rear sides of the groove 44, annular grooves 45 are formed in the outer periphery of the output shaft 31 over the entire circumference thereof. An O-ring 46 and a seal ring 47 are fixedly fitted in each of these grooves 45. Thus, a pressure oil chamber 48 is formed. A branch pipe 49 branching from the conduit 39 and communicating with the pressure oil chamber 48 is provided with a hydraulic reaction control valve 52 controllable by a control unit 51 in accordance with the vehicle speed detected by a vehicle speed sensor 50.
The periphery of the input shaft 33 is formed with a plurality of furrows 53 extending axially of the shaft, arranged at a specified spacing circumferentially thereof and opposed to the respective holes 32. Each of the furrows 53 is defined by a flat bottom surface 53aand opposite side surfaces 53b extending radially outward and inclined circumferentially outward.
The pressure oil is introduced into the pressure oil chamber 48 in accordance with the vehicle speed, whereby the spherical protuberances 41a at the inner ends of the plungers 41 are pressed against the inner surfaces of the respective furrows 53 in the periphery of the input shaft 33 to increase the steering torque required for rotating the input shaft 33 about its axis. The reaction pressure to be applied to the plungers 41 increases with the steering pressure to be given to the power cylinder.
FIG. 14, the solid lines represent the relationship between the steering torque and the steering pressure, i.e., steering characteristics, of the vehicle during running at a high speed. Stated more specifically, suppose the steering wheel is turned to rotate the input shaft 33, for example, counterclockwise in FIGS. 12 and 13. In this case, the steering torque only increases initially. When the input shaft 33 has rotated through a predetermined angle, the torsion bar 34 is twisted, increasing the steering pressure within one of the pressure oil chambers of the power cylinder At the same time, the reaction pressure to be given to the plungers 41 and the steering torque increase linearly (see FIG. 14, line A). At this time, each of the plungers 41 slightly slides in the hole 32 radially outward, with its protuberance 41a sliding on the bottom surface 53a of the furrow 53 rightward in FIG. 13.
When the input shaft 33 is further rotated, the torsion bar 34 is twisted to a greater extent to give a greater steering pressure. However, an unillustrated pressure control valve functions to keep the reaction pressure constant and thereby prevent the increase in the steering torque (see FIG. 14, line B). At this time, the spherical protuberance 41a of each plunger 41 slides up the inclined side surface 53b of the furrow 53, moving the plunger 41 radially outward within the hole 32.
Upon the steering pressure reaching a point X in FIG. 14, an unillustrated relief valve functions so as not to increase the steering pressure above the value. The output shaft 31 starts to rotate counter-clockwise to decrease the steering torque (see FIG. 14, line C). This movement slightly slides down the protuberance 41a of the plunger 41 along the inclined side surface 53b of the furrow 53 to slightly slide the plunger 41 radially inward in the hole 32.
After the steering torque has reached a point Y in FIG. 14, the steering pressure only diminishes (see FIG. 14, line D). At this time, the output shaft 31 further rotates counterclockwise, causing the plunger protuberance 41a to slide down the inclined side surface 53b of the furrow 53 to the lower end thereof to move the plunger 41 radially inward inside the hole 32.
The steering pressure decreases to a point Z, whereupon the reaction pressure also decreases with the decrease of the steering pressure, and the steering torque decreases linearly along with the steering pressure and the reaction pressure (see FIG. 14, line E). At this time, the plunger protuberance 41a slides on the furrow bottom surface 53a leftward in FIG. 13, and the plunger 41 slides radially inward within the hole 32. In this way, the front wheels are steered through a specified angle.
In a low-speed running state, no reaction pressure is applied to the plungers 41, and the input shaft 33 is rotated with a small steering torque to twist the torsion bar 34 and supply the pressure oil to the power cylinder.
However, the conventional power steering device has a problem. Since each plunger 41 is made entirely of metal, a great frictional force occurs when the spherical protuberance 41a slides on the bottom surface 53a and the side surface 53b of the furrow 53 and also when the plunger 41 slides axially of the hole 32, with the result that the steering characteristics shown in FIG. 14 involve great hysteresis to give an impaired steering feeling.
Further in an equilibrated state wherein no oil pressure is applied to the plunger 41 as when the vehicle is at a stop or is running at a low speed, another problem is encountered in that the plunger 41 moves within the hole 32 to produce a striking noise when the plunger 41 is forced against the input shaft 33 on steering. Additionally, the device has the problem that upon the application of oil pressure to the plunger 41 in the equilibrated state, the plunger 41 abruptly moves in the radial direction into impinging contact with the input shaft 33 to give off a striking noise. Especially, the plunger 41, which is made of metal and has a great weight, produces a great striking noise.
An object of the present invention is to solve the foregoing problems and to provide a vehicle speed responsive hydraulic power steering device which gives a more excellent steering feeling than the conventional device.
Another object of the invention is to provide a vehicle speed responsive hydraulic power steering device wherein the plungers are adapted to strike on the input shaft with a diminished noise.
Other objects of the invention will be readily understood from the following description.
The hydraulic power steering device responsive to the speed of a vehicle and embodying the present invention comprises a housing, an output shaft disposed in the housing, an input shaft disposed in the housing and coaxial with the output shaft, a torsion bar provided between the output shaft and the input shaft, a control valve provided around the input shaft for controlling the amount of pressure oil to be supplied from a hydraulic pump to a power cylinder in accordance with the torsion of the torsion bar, and plungers each slidably fitted in a hole formed in the output shaft, each of the plungers being adapted to be pressed at its forward end against the input shaft by oil pressure in accordance with the vehicle speed to obtain a reaction, at least one of the portion of the plunger slidable in contact with the inner peripheral surface of the output shaft defining the hole and the portion of the plunger to be pressed into contact with the input shaft being made of a synthetic resin.
Examples of plungers for use in the power steering device described above includes a plunger which is entirely made of a synthetic resin; a plunger comprising a synthetic resin body having a portion slidable in contact with the hole-defining inner peripheral surface of the output shaft, and an input shaft contacting metal ball fixed in the synthetic resin body; a plunger comprising a synthetic resin body having a portion slidable in contact with the hole-defining inner peripheral surface of the output shaft, and an input shaft contacting metal ball rollably attached to the synthetic resin body; and a plunger comprising a synthetic resin body, and an annular metal body secured to the periphery of the synthetic resin body and having an outer peripheral surface slidable in contact with the hole-defining inner peripheral surface of the output shaft, one end of the synthetic resin body closer to the input shaft being projected beyond one end of the annular metal body closer to the input shaft to provide a protuberance for contact with the input shaft.
With the vehicle speed responsive hydraulic power steering device of the present invention, at least one of the plunger portion slidable in contact with the hole-defining inner surface of the output shaft and the plunger portion to be pressed into contact with the input shaft is made of a synthetic resin. This reduces the sliding friction between the plunger and the furrow bottom surface and side surfaces of the input shaft and/or the sliding friction between the plunger and the hole-defining surface of the output shaft. Moreover, the plunger is smaller in weight than the conventional one which is made entirely of a metal material. Consequently, the hysteresis of the steering characteristics is smaller than in the prior art, permitting the rotation of the output shaft to follow the rotation of the steering wheel more effectively to give an improved steering feeling. The lightweight plunger further diminishes the noise to be produced by the striking contact of the plunger with the input shaft due to the movement of the plunger.
The present invention will be described in greater detail with reference to the accompanying drawings.